How can I apply a ripple effect on the screen/camera (not on puddles or rivers)
You can also use dynamic paint to obtain many ripple effects in a more flexible way than using the compositor uniquely :



The usage may be different from the one you could obtain with the compositor, but this approach can interest you.

Here is the settings :


A plane, with a subsurface modifier (set to 'simple' in order to keep it angular)
And a dynamic paint modifier





In the physics panel of the plane, set the dynamic paint to canvas, then to wave and eventually adjust the parameters :





Make another mesh (or several, here I used a sphere) and add it also a dynamic paint modifier, but this time set it to brush :




And you are done : simply move the brush object into the canvas surface and it makes ripples during the animation (you need to run the animation to obtain it).

You can mix it with your animation, using for instance a glass shader, or putting it in another layer and merge it with the compositor.